I would have liked to use the word "tiny" in the title - but, unfortunately, that denotes a particular Atmel product.
I am interested in radio control for NGauge 1:148 scale model trains.
I discovered recently that the Deltang Rx6x radio-control receivers use an Atmega328. They also contain a Cypress CYRF6936 2.4GHz radio transceiver and a motor control bridge. The total size of the device is 12 x 22 mm. Deltang make smaller Rx4x receivers but they don't use Atmel MCUs. http://www.deltang.co.uk/
As far as I can see this is the smallest Arduino + wireless platform. The Moteino measures 33 x 22 and doesn't include the bridge circuitry - almost 3 times bigger, and won't fit in NGauge model trains. Deltang is a UK product and Moteino is a US product.
The standard Deltang products are designed to be controlled with regular 2.4Ghz radio control signals. When I discovered that the Rx6x modules are based on Atmega328 MCUs I was interested to see if they could be "converted" to Arduino devices. I purchased two of them and also got a great deal of support from David Theunisson of Deltang. I believe that if the modules are operated as "Arduino devices" they offer much more facilities than they do in the "airplane" R/C mode. For example, two-way communication is immediately available (as it is with Moteino), and many trains could be controlled with a single "transmitter" module.
Of course Arduino folk might find all sorts of other applications for very small wireless-equipped Arduino modules.
Uploading the Arduino bootloader was easy following the instructions here http://arduino.cc/en/Tutorial/ArduinoToBreadboard for the "Minimal Circuit (Eliminating the External Clock)". With the bootloader in place scripts can be uploaded in the usual way using the Tx, Rx and Reset connections with the 328 chip removed from the Uno board. NOTE that the Atmega328 on the Rx6x board works at 3v so can't be connected directly to Uno output signals.
The CYRF6939 transceiver is widely used in commercial products but not so much by hobbyists so it took me a while to gather information about how to set it up. There are a great many options.
I have prepared a short pdf document giving more details and a demonstration sketch that works with a pair of Rx61a modules. They can be downloaded from here https://docs.google.com/file/d/0B0hhHjM70n1TNkllamVCRFBQMGM/edit?usp=sharing
I have no connection with Deltang other than as a satisfied customer.
PS is there anywhere on the Arduino site where there could be a permanent link to the product?